Spread mustard on one side of six slices of bread. Layer the ham, cheese and tomato over mustard; top with remaining bread. Butter the outsides of the sandwiches; cut in half. Arrange in a greased 13-in. x 9-in. baking dish. In a small bowl, whisk the eggs, milk and pepper; pour over sandwiches. Cover and refrigerate overnight. Remove from the refrigerator 30 minutes before baking. Bake, uncovered, at 375° for 30 minutes or until a knife inserted in the center comes out clean. Yield: 6 servings.